Raman scattering from vibrational modes inSi46clathrates

Room-temperature Raman-scattering spectra are reported for the type-II superconductors ${\mathrm{M}}_x{\mathrm{Ba}}_y{\mathrm{Si}}_{46}$ (M=Na,K) that were recently shown to exhibit $T_c$’s ∼3.5 K. These spectra are compared to those of ${\mathrm{Na}}_8{\mathrm{Si}}_{46}$ and ${\mathrm{K}}_7{\mathrm{Si}}_{46}$ clathrates that exhibit normal metallic behavior down to 2 K. In the ${\mathrm{Si}}_{46}$ system, fifteen of the eighteen Si related first-order Raman frequencies predicted by group theory have been detected, and the frequencies are found to be sensitive to the particular dopants. The Raman linewidths observed for the ${\mathrm{M}}_x{\mathrm{Ba}}_y{\mathrm{Si}}_{46}$ system are comparable to those observed for ${\mathrm{Na}}_8{\mathrm{Si}}_{46}$ and ${\mathrm{K}}_7{\mathrm{Si}}_{46}.$ The data, taken collectively, suggest that the line broadening in the metallic Si clathrates is due to important contributions from both the electron-phonon interaction as well as from a random filling of the Si cages and charge-transfer disorder.